The hardness of a water sample (in terms of equivalents of CaCO₃) containing 10^-3 M CaSO₄, is  ---------$\text{ (Molar mass of }{\mathrm{CaSO}}_4=136 \mathrm{g} {\mathrm{mol}}^{-1}\text{ ) }$

Hardness of water sample can be calculated in terms 
of ppm concentration of CaCO₃. 
Given, molarity = 10^-3 M 
i.e. 1000 mL of solution contains 10^-3 mole of CaCO₃. 
Hardness of water = ppm of CaCO₃

                              $=\frac{{10}^{-3}\times 100}{1000}\times {10}^6=100 \mathrm{ppm}$